To most of us, wheels seem pretty intuitive. You've probably known from an early age, for example, that circular or rounded things roll more easily than boxy or angular things. Knowing this, it's hard to imagine that our earliest ancestors did not come to similar realizations on their own — and yet, the first wheels don't show up in history until around 3500 BC. By that time, people were already writing, farming, and even casting metal alloys. So what was so damn elusive about creating a wheel?

Now, we're obviously approaching this question of design over five thousand years after-the-fact; we've had a long time to improve upon the wheel, and (at least for an overwhelming majority of Earth's population) the wheel is an integral part of our daily lives. In other words, we've been studying the key to the exam on wheels for thousands of years — of course it would seem obvious to us.

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And yet, many archaeologists and anthropologists think that the function and utility of a wheel likely seemed obvious to our ancestors, as well. The real challenge was in figuring out how to make that wheel operate as part of a larger system of moving parts — a primitive wagon, for example. As anthropologist David Anthony explained to Life's Little Mysteries, the real stroke of brilliance was the wheel-and-axle concept. But even the idea for this design probably came long before the wheel was actually invented. Life's Little Mysteries explains:

To make a fixed axle with revolving wheels, Anthony explained, the ends of the axle had to be nearly perfectly smooth and round, as did the holes in the center of the wheels; otherwise, there would be too much friction for the wheels to turn. Furthermore, the axles had to fit snugly inside the wheels' holes, but not too snugly - they had to be free to rotate.

In other words, the wheel's widespread implementation most likely wasn't delayed by a lack of inspiration (hey, a cylinder rolling on its edge would be a great way to lug stuff from point a to point b), but a lack of proper tools to see that inspiration realized (gee, i sure wish I had access to a powersaw).

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But this is where experts stop seeing eye-to-eye. Once someone had all the starting materials (thick-trunked trees, most likely) and tools (metal implements like chisels and gouges), were they designing rolling rigs right off the bat, or were they developed in stages?

According to Anthony, "the sensitivity of the wheel-and-axle system" precludes the possibility that it was developed in phases. This design, he claims, was an "all-or-nothing" structure that probably happened once, in one place, before rapidly spreading throughout Europe, Asia, and the Middle East.

TURN TWO: Add a plank between the load and the rollers, reducing friction.

TURN THREE: Stick Blades - like a sledge - under the board to reduce friction further.

TURN FOUR: The blades create grooves in the tree trunks that stabilize the load.

TURN FIVE: The tree trunks are hollowed between the grooves so that different-sized sledges can fit on top (and an accidental axle and wheel pair is formed).

TURN SIX: Pegs are attached to the bottom of the blades, in front of and behind the position of the axle, so that the axle is contained underneath the load and there is no longer the need to repeatedly place rollers at the front (that must have been a great day!).

TURN SEVEN: To make the cart stronger, the axle is fed through holes fashioned in the blades of the sledge.

TURN EIGHT: Ta-dah!

Read more about the all-in-one theory and the stepwise model for the wheel's invention on Life's Little Mysteries and NPR, respectively.